Hydroformylation is a well known process in which an olefin is reacted with carbon monoxide and hydrogen in the presence of a catalyst to form aldehydes and/or alcohols containing one carbon atom more than the feed olefin. In high pressure hydroformylation processes, i.e. at pressures of 100 bar gauge or above, the catalyst is typically a homogeneous metal carbonyl complex, typically of a transition metal of Group VIII of the Periodic Table and carbon monoxide. Within the metals of this Group VIII, cobalt and rhodium are the best known for their hydroformylation activity, but others including palladium, iridium, ruthenium and platinum are also suitable. Cobalt is particularly preferred for the high pressure hydroformylation of olefinic feedstocks that are rich in branched and internal olefins. The cobalt carbonyl catalyst typically produces oxygenated product mixtures that are richer in the usually more desired less branched isomers, as compared to the carbonyl catalysts of the other metals, in particular of rhodium.
The present invention is concerned with the recovery and recycle of cobalt carbonyl catalyst from the hydroformylation reaction, also known as the Oxo or the oxonation reaction.
The starting liquids that are involved in high pressure hydroformylation comprise olefins which may be mixtures of olefins such as those obtained from olefin oligomerisation units. For example the olefins may be mixtures of C5 to C12 olefins obtained by the phosphoric acid or zeolite catalysed oligomerisation of mainly C3 and C4 olefins and mixtures thereof. C5 olefins may also be present during oligomerisation, as well as traces of ethylene. Where olefin mixtures are used as feed for hydroformylation, they may have been fractionated to obtain relatively narrow boiling cut mixtures of mostly the appropriate carbon number for the production of aldehydes and alcohols with the desired carbon number. Alternatively the olefins may be obtained by other oligomerisation techniques. Such techniques include the dimerisation or trimerisation of butene using a nickel-based or nickel oxide catalyst, like the Octol® process or the process described in U.S. Pat. No. 6,437,170. Others include oligomerisation processes for ethylene, propylene, pentenes and/or butenes, preferably single carbon number feedstocks and more preferably the unbranched, even more preferably terminal olefins such as butene-1, using a nickel salt and involving di-alkyl aluminium halides, like the range of Dimersol® processes. Yet other processes employ a zeolite or a molecular sieve oligomerisation catalyst for the oligomerisation of propylene and/or butenes and/or pentenes. The olefin products of these processes are typically branched and contain relatively low amounts of linear olefin isomers, typically less than 10% wt.
The olefins may also be obtained from ethylene growth processes, such as the SHOP or the Ziegler processes, in which case they are often straight chain, preferably terminal olefins, and are called linear alpha olefins or normal alpha olefins. The SHOP process may include a metathesis step, in which case also uneven carbon numbers may be produced. The olefins from ethylene growth may have C6, C8, C10 or C12, or even higher carbon numbers such as up to C14, C16, C18 or even C20, or they can be mixtures obtained from the Fischer-Tropsch process for the conversion of synthesis gas to hydrocarbons, which generates olefins of a range of carbon numbers, primarily containing terminal olefins but which may show some side branches along their longest alkyl chain, and which may also contain some internal olefins, linear and branched. In this case, also the higher carbon numbers may be useful starting liquids. Fischer-Tropsch olefins suitable for high pressure hydroformylation are disclosed in EP 835 234, but many other disclosures in this field may readily be found. The Fischer-Tropsch process uses syngas as the starting material, and suitable sources thereof are disclosed further below in a separate paragraph.
The starting materials for the olefin oligomerisation processes mentioned above may be obtained from fluid catalytic cracking (FCC), from the steam or thermal cracking of gasses such as ethane and propane, of liquids such as liquefied petroleum gasses (LPG), of naphtha, of gasoil or heavier distillate, or even of whole crude. The oligomerisation starting material may also come from oxygenate-to-olefin processes, and from paraffin dehydrogenation processes.
The gases that are involved in high and medium pressure hydroformylation reactions include carbon monoxide and hydrogen, frequently supplied in a mixture that is known as synthesis gas or “syngas”. Syngas can be obtained through the use of partial oxidation technology (POX), or steam reforming (SR), or a combination thereof that is often referred to as autothermal reforming (ATR). Thanks to the water-gas-shift reaction for supplying the hydrogen, it can be generated from almost every carbon containing source material, including methane, natural gas, ethane, petroleum condensates like propane and/or butane, naphtha or other light boiling hydrocarbon liquids, gasoline or distillate-like petroleum liquids, but also including heavier oils and byproducts from various processes including hydroformylation, and even from coal and other solid materials like biomass and waste plastics, as long as these provide a carbon source and can be brought into the reaction zone. When using liquid feeds for syngas generation, a steam reformer may involve a pre-reformer to convert part of the feed to methane or other light hydrocarbon gasses before entering the actual reformer reaction. The use of coal as feedstock for generating syngas is well known, preferably via the POX or ATR route. Such syngas may be fed directly as syngas feed for hydroformylation, but also as a feed to a Fischer-Tropsch process to generate the olefin feedstocks for the hydroformylation reaction. The latter is of interest for geographic regions where the other above-mentioned carbon sources, in particular oil and gas, are less abundant.
In the production of higher alcohols, the metal catalyst is used for the reaction of the olefins with synthesis gas. After completion of the hydroformylation (oxonation) reaction, the metal catalyst must be removed from the reaction products. For economic reasons the metal is preferably recycled for use as the catalyst in the oxonation reaction. For environmental reasons it is important that the level of metal in any waste streams from the process be minimized. More preferably, the catalyst cycle does not involve a waste stream.
The Group VIII transition metals such as cobalt are potentially hazardous and expensive materials. They may also impair downstream process steps, such as hydrogenation of the aldehyde-containing hydroformylation product to form the alcohol. Accordingly there are both environmental and economic benefits to be realized by improving the catalyst metal recovery and recycle from hydroformylation reactions.
The Group VIII transition metal species that is generally the active form of the catalyst for hydroformylation is a carbonyl compound. In the case of cobalt, it is a cobalt carbonyl and is typically hydr(id)ocobalt (tetra)carbonyl, HCo(CO)4. Under reaction conditions, it is believed that the following equilibrium reaction occurs, and under high pressure and temperature conditions the equilibrium is significantly shifted to the left.2HCo(CO)4<--------->Co2(CO)8+H2  (1)
The hydroformylation catalyst is typically homogeneous, i.e. dissolved in a reaction phase, and more typically in the organic reaction phase. Hence, significant amounts of it typically remain in the product of the hydroformylation reaction, and must be removed therefrom and preferably recycled.
Several technologies for recovery and recycle of a cobalt catalyst from the hydroformylation reaction are known. The commercially more important technologies for operating an oxo catalyst cycle are described by J. Falbe in “New Synthesis with Carbon Monoxide”, Springer-Verlag, 1980, in particular on pages 158 to 176. A more recent review may be found in Beller et al., “Progress in hydroformylation and carbonylation”, Journal of Molecular Catalysis, A: Chemical, 104 (1995) pages 17-85.
One family of hydroformylation catalyst cycles involves the substantially complete decomposition of the cobalt carbonyl to a water soluble salt, having cobalt as the cation, and preferably with the anion of a low molecular weight organic acid, while simultaneously extracting the cobalt salt into an aqueous phase for separation from the organic hydroformylation product.
These techniques may use an oxidant, e.g. an oxygen-containing gas or air, such as those being described in U.S. Pat. Nos. 2,547,178 (Spence), 3,520,937 (Moell et al), 3,929,898 (Nienburg et al) and 6,723,884 (Grenacher et al), and are often called “air-demetalling”. The technique is based primarily on the following reaction, shown here for acetic acid, during which the cobalt moves from the organic to the water phase:2HCo(CO)4+3/2O2+4CH3COOH------>2Co(CH3COO)2+3H2O+8CO  (2)
The technique may alternatively use the dilute acid solution without an oxidant, such as described in GB 702 950, FR 1 089 983 or U.S. Pat. Nos. 2,744,936 and 2,841,617 (Mertzweiller), in which case it is typically called “airless demetalling”. This alternative takes advantage of the following so-called “disproportionation” reaction:3Co2(CO)8<-------->2Co2++4Co(CO)4−+8CO  (3)
Because Co2(CO)8 is practically insoluble in water, reaction (3) occurs at the oil/water interface. The equilibrium of reaction (3) is influenced by the presence of CO. Under sufficiently strong acidic reaction conditions, this may be followed by:Co(CO)4−+H+<-------->HCo(CO)4  (4)
The undissociated cobalt carbonyl product from reaction (4) may then move again to the organic phase, and a reaction loop over reactions (1), (3) and (4) may be formed, which ultimately destroys all cobalt carbonyls and ends up with all cobalt as Co2+, similar to the result of the air demetalling technique described hereinbefore, but now without the help of an oxidant.
The technique described in U.S. Pat. No. 2,744,921 uses such an airless demetalling step. The cobalt-containing water is routed to a catalyst plant where it is mixed with an olefin solution containing a sodium salt of a heavier organic acid, and air is introduced into that mixture to ensure all cobalt carbonyls are oxidised to Co2+ before or at the same time as converting the water soluble cobalt salt to a cobalt soap of the heavier organic acid, which is then transferable to the hydroformylation reaction in an organic carrier.
In these techniques, substantially all of the cobalt carbonyl species are destroyed. In these disclosed techniques, the cobalt is oxidised from its (−1) oxidation state in HCo(CO)4 and/or its (0) oxidation state in Co2(CO)8 into its Co2+ oxidation state in the water soluble cobalt formate or acetate. Upon recycling, and it is believed also before any hydroformylation can occur, the cobalt must be reconverted to the active carbonyl form by reacting it with carbon monoxide and optionally also hydrogen in the so-called “preforming” reaction, also called metal carbonylation.
This preforming reaction may be performed in the hydroformylation reactor itself, or in an additional reactor upstream thereof, which is typically called a preformer or preforming reactor. Such preforming reactor typically operates at high temperature and high pressure, and adds significant complexity to the process. If the preforming reaction is performed in the presence of olefin feed, such as in the hydroformylation reactor itself, the preforming reaction may be impaired by components present in that feed, such as di-olefins, resulting in a delayed initiation of the hydroformylation reaction, particularly noticeable at start-up.
Many of these techniques have another drawback, i.e. that they are limited by the water solubility of the cobalt salt, which limits the amount of catalyst metal that can be made available to the hydroformylation reactor, or alternatively increases the volume of water that needs to be passed through the reactor and thereby reduces the volumetric efficiency of the high pressure reactor, which is typically an expensive equipment item. An alternative to overcome this limitation is to add an extraction step between the preformer and the hydroformylation reactor, such as described in U.S. Pat. No. 3,929,898, so that the metal carbonyl is extracted into an organic carrier for transfer to the hydroformylation reactor. This again adds significant further complexity and investment cost to the catalyst cycle.
Because of these drawbacks, catalyst cycles retaining the cobalt in the carbonyl form have been searched for and identified.
One example is the so-called “Kuhlmann cycle”, described for example in U.S. Pat. No. 3,188,351 (Lemke), in which, by contact with a dilute sodium base, a sodium salt of the cobalt carbonyl is formed and separated in an aqueous solution from the hydroformylation product, the so-called “carbonylate” solution. By adding later a strong acid to this carbonylate, volatile HCo(CO)4 is formed, which may be stripped at low pressure from the liquid and carried with the stripping gas to an absorber for absorption into the feed olefin. Alternatively, the acidified carbonylate is contacted with an organic, such as the feed olefin, for extraction of the undissociated HCo(CO)4 and recycling with the olefin to the oxo reaction. This technique can be made highly efficient in maintaining the cobalt as carbonyl throughout the catalyst cycle, and avoids the need for large volumes of water to pass through the hydroformylation reactor. Drawbacks of this technique are the consumption of chemicals and the environmental burden related to the disposal of the dilute acid stream that is left over after the stripping step.
An alternative is the so-called “Cobalt Flash” technique, which is described in U.S. Pat. No. 4,625,067 (Hanin). In this technique, volatile HCo(CO)4 is stripped directly from the organic liquid hydroformylation product and absorbed into the olefin feed for recycle to hydroformylation. Only a part of the cobalt may typically be recovered by stripping. Typically a smaller portion of the cobalt in the hydroformylation product converts to its water soluble salt of an acid that is provided, typically to cobalt formate, and upon separation and concentration of the solution thereof, and optionally a preforming step, such as proposed in WO 93/24436, this cobalt may also be recycled to hydroformylation. Several variations of this technique are known, such as in combination with an airless demetalling step as described in U.S. Pat. No. 5,410,090 (Beadle et al), or in a number of alternative combinations with an air demetalling step as described in U.S. Pat. No. 5,327,105 (Summerlin). An improved cobalt absorption step is disclosed in U.S. Pat. No. 5,354,908, offering a more concentrated cobalt containing olefin stream for feeding to the hydroformylation reaction. Again, no large volumes of water need to be passed through the hydroformylation reactor. These “Cobalt Flash” techniques provide significant environmental and operational benefits, as they may be operated with little or no byproduct waste streams. However, they are relatively complex techniques.
Intermediate alternatives are also known, in which only a part of the cobalt carbonyl is decomposed and a significant remainder of the cobalt is retained in its carbonyl form.
U.S. Pat. No. 4,255,279 (Spohn et al) describes a cobalt hydroformylation catalyst cycle wherein the cobalt is removed from the crude oxo product by dual demetalling, i.e. in two consecutive steps. In the first step, the oxo product is treated with an aqueous solution of a Co2+ salt of an acid to form an aqueous phase containing Co[Co(CO)4]2, ideally with only the following neutralisation and extraction reaction taking place:2HCo(CO)4+Co(CH3COO)2<--------->Co[Co(CO)4]2+2CH3COOH  (5)
In a subsequent second demetalling step, the demetalling of the oxo product is completed by an air-demetalling step to form an aqueous solution of a Co2+ salt of an acid, using reaction (2) as explained hereinbefore.
The aqueous solution from the second demetalling step is then used as the feed to the first demetalling step. The aqueous phase from the first demetalling step, containing Co[Co(CO)4]2, is separated and optionally treated with synthesis gas for preforming any excess Co2+ salt that may be present to make more Co[Co(CO)4]2. A portion of the cobalt carbonyls are then extracted from the aqueous phase that contains the Co[Co(CO)4]2, at high pressure and into an organic solvent, which organic product is then passed to the oxo reactors as catalyst in an active non-aqueous form. The aqueous phase left over from the high pressure extraction and containing all the Co2+ plus a significant portion of the cobalt carbonyls from the starting Co[Co(CO)4]2 is returned to the second demetalling step, where all cobalt carbonyls have to be destroyed in order to prevent them leaving with the organic reaction product. This creates a significant excess of Co2+ presence and circulation in the catalyst cycle. The high pressure extraction step as part of the process of U.S. Pat. No. 4,255,279 avoids the risk of flooding the oxo reactors, and eliminates corrosion concerns associated with the injection of an aqueous cobalt solution directly into the oxo reactors. However, the process of U.S. Pat. No. 4,255,279 has the disadvantage that only a part of the cobalt in the aqueous phase from the first demetalling step reaches the oxo reactors and becomes available as catalyst in the hydroformylation reaction. The remaining part returns to demetalling, first to the second step for all cobalt to be converted to Co2+, from which it is recycled to the first step in order to provide all the Co2+ needed to participate as Co(CH3COO)2 in the extraction reaction (5).
The process of U.S. Pat. No. 4,255,279, in practice, leads to an excess Co2+ being present over that needed for the Co[Co(CO)4]2 formation. This excess Co2+ is formed firstly because cobalt carbonyls remain in the aqueous phase returning from the high pressure extraction step, and secondly because not all cobalt carbonyls that are present in the Oxo product are extracted in the first demetalling step. U.S. Pat. No. 4,255,279 needs an additional and complex high pressure preforming (and extraction) step in order to correct for this excess Co2+ formation and to allow not even two thirds of the cobalt in the cobalt water recycle to the preformer to reach the oxo reactors.
Using olefins as the organic extraction liquid, as is preferred in U.S. Pat. No. 4,255,279, should make that process even more complex, because under the preforming conditions the hydroformylation reaction should also take place, and it is strongly exothermic and needs to be controlled. Performing the extraction with the olefins is at the 77° F. at which the equilibrium distribution of HCo(CO)4 is shown, makes the process even more complex by adding an extra high pressure cooler between preformer and extraction. U.S. Pat. No. 4,255,279 teaches away from recycling an aqueous stream of Co[Co(CO)4]2 to the oxo reactors as catalyst, as was disclosed 25 years earlier in U.S. Pat. No. 2,757,205, because of the risk of flooding the reactors.
The cobalt catalyst cycle in U.S. Pat. No. 2,757,205 (Mertzweiller et al) uses carbonyl extraction and “airless demetalling” in a single demetalling step, treating the hydroformylation product with a dilute aqueous acid in presence of synthesis gas. The resulting aqueous solution of Co[Co(CO)4]2 and cobalt acetate is recycled directly and entirely to the hydroformylation reaction. A concentration step is proposed on this recycle, but due to the presence of cobalt carbonyls this is not able to provide a cobalt-free water side stream. The demetalling step in U.S. Pat. No. 2,757,205 requires careful control of temperature, a definite partial pressure of synthesis gas and hence of CO, and a residence time of 30-120 minutes. These conditions represent a balancing compromise, on one hand to promote the slow interface reaction (3), which is equilibrium limited, helped by the use of high temperature but impaired by the CO partial pressure, and on the other hand to inhibit any carbonyl breakdown reactions thanks to the high CO partial pressure. The aldehyde product after treatment therefore still contains significant levels of catalyst metal, such as 54-250 ppm of cobalt. A further hot water washing step is therefore needed in U.S. Pat. No. 2,757,205 to substantially complete the decobalting of the hydroformylation reaction product. Due to the high levels of cobalt remaining after the first demetalling step, caused by the equilibrium at the process conditions, the hot water washing step only reaches a cobalt level of 8 ppm by weight, which is unacceptably high in todays operations. This washing step introduces significant amounts of water into the catalyst cycle, which has to be removed from the aqueous part of the cycle somewhere, in order to maintain a constant water inventory. The water from this washing step may be employed as the diluent for the organic acid used in the demetalling, but should first be concentrated to maintain the water balance of the system. The process in U.S. Pat. No. 2,757,205 has the advantage that all the cobalt in the recycle stream from the decobalting settler reaches the hydroformylation reaction. Because of the need for high residence times and intense mixing however, this process has the disadvantage that it requires large hold-up volumes of organic liquid in the decobalting section. This requires large equipment sizes and adds to safety concerns for operating the overall process, to a level that is unacceptable for a world scale alcohol plant of today.
There remains therefore still a need to provide a simpler, more effective and volume-efficient catalyst cycle for hydroformylation reactions. We have found that a significant improvement in volume efficiency may be achieved in the demetalling step. We have also found that, in the recycle of the metal carbonyl to the hydroformylation reaction, the need for a complex preforming and carbonyl extraction step, before and separate from the hydroformylation, may thereby also be avoided.